The importance of good roads and highways has been appreciated since the time of the Roman Empire. By about 300 B.C., the first section of the Appian Way extending from Rome to Capua was built. Some of the more than 50,000 miles of roadway ultimately built in the Roman Empire was constructed with heavy stone. However, not much progress was made in the art of road construction from the era of the Roman Empire until the development of the motor vehicles, such as automobiles and trucks.
For centuries, stone blocks, wood blocks, vitrified brick and natural asphalt (bitumen) have been used to pave roads and highways. However, at the beginning of the automobile era, most rural roadway surfacing consisted of broken stone or gravel. Such roads were often rough, dusty and clearly inadequate for modern automobile and truck traffic.
Today, the United States has the most extensive highway system in the world with about 2,000,000 miles of paved road. Napoleon realized the importance of roadway systems and built such a system in France which today has the second most extensive system of paved roadways in the world covering about 500,000 miles. Germany, Japan, Great Britain, India and Australia also currently have systems of paved roads which extend well over 100,000 miles. In addition to these public roadways, there are countless paved driveways and parking lots all over the world.
Today, roads, highways, driveways and parking lots are often paved with asphalt concrete. Pavement can be made with asphalt concretes which are dust-free, smooth and which offer the strength required for modern automobile and heavy truck traffic. Asphalt concrete is generally made by mixing aggregate (sand and gravel or crushed stone) with the proper quantity of an asphalt cement at an elevated temperature. The hot asphalt concrete is then placed by a layering machine or paver on the surface being paved and thoroughly rolled before the asphalt concrete mixture cools. The asphalt concrete is normally applied at a thickness varying from about 25 to about 100 millimeters.
Asphalt concrete pavements can be made to be very smooth which offers outstanding frictional resistance for vehicles operating thereon. Such asphalt concrete pavement can also be repaired simply by adding additional hot asphalt concrete to holes and other types of defects which develop in the surface. Asphalt concrete pavements can also be upgraded easily by adding additional layers of hot asphalt concrete to old surfaces which are in need of repair.
Even though asphalt concrete offers numerous benefits as a paving material, its use is not trouble-free. One major problem encountered with asphalt concrete pavements is the loss of the adhesive bond between the aggregate surface and the asphalt cement. This breaking of the adhesive bond between the asphalt cement and the aggregate surface is known as "stripping." The stripping of asphalt binder from aggregate surfaces results in shorter pavement life and necessitates the implementation massive annual highway maintenance programs. Reduction of this stripping tendency is of great interest when trying to improve the condition of roads while lowering maintenance costs.
Over the years, various methods have been developed to reduce stripping tendencies. For instance, amines and lime are known to act as anti-stripping agents and are frequently applied to the surface of the aggregate prior to mixing it with the asphalt cement in making asphalt concrete. U.S. Pat. No. 5,219,901 discloses a technique for reducing stripping tendencies which involves coating the aggregate with a thin, continuous film of a water-insoluble high molecular weight organic polymer, such as an acrylic polymer or a styrene-acrylic polymer.
U.S. Pat. No. 5,262,240 discloses an excellent technique for providing aggregate with a high level of resistance to stripping by water, which comprises: (1) mixing the aggregate with latex to form a latex/aggregate mixture which is comprised of from about 0.005 weight percent to about 0.5 weight percent dry polymer; (2) heating the latex/aggregate mixture to a temperature which is within the range of about 66.degree. C. to about 232.degree. C.; (3) maintaining the latex/aggregate mixture at said elevated temperature for a time which is sufficient to reduce the moisture content of the latex/aggregate mixture below about 0.7 weight percent and to allow the polymer in the latex to crosslink on the surface of the aggregate to produce the coated aggregate.
The technique disclosed by U.S. Pat. No. 5,262,240 can be used to treat most aggregates with excellent results. However, its use on some clay-bearing aggregates which undergo water absorption and can result in a loss of compressive strength and modulus. This, of course, results in the asphalt concrete made therewith having less desirable physical characteristics. Thus, the technique disclosed in U.S. Pat. No. 5,262,240 is not optimal for use on some clay-bearing aggregates. There is, accordingly, a current need for a better technique for coating aggregate with latex which does not result in reduced compressive strength and reduced modulus.